Discovery of Three Distant, Cold Brown Dwarfs in the WFC3 Infrared Spectroscopic Parallels Survey

TL;DR

This paper reports the discovery of three distant, cold brown dwarfs, including a probable Y dwarf, using infrared spectroscopy, and compares their numbers with models to understand the brown dwarf population.

Contribution

It presents the first spectroscopic confirmation of three distant late-type brown dwarfs, including a Y dwarf, and analyzes their implications for the brown dwarf mass function.

Findings

Most distant spectroscopically confirmed T/Y dwarfs at ~400 pc

Number of cold dwarfs aligns with a flat to mildly rising mass function

Detection of a Y dwarf suggests possible abundance of very late-type brown dwarfs

Abstract

We present the discovery of three late type (>T4) brown dwarfs, including a probable Y dwarf, in the WFC3 Infrared Spectroscopic Parallels (WISP) Survey. We use the G141 grism spectra to determine the spectral types of the dwarfs and derive distance estimates based on a comparison with nearby T dwarfs with known parallaxes. These are the most distant spectroscopically confirmed T/Y dwarfs, with the farthest at an estimated distance of ~400 pc. We compare the number of cold dwarfs found in the WISP survey with simulations of the brown dwarf mass function. The number found is generally consistent with an initial stellar mass function dN/dM \propto M^{-\alpha} with \alpha = 0.0--0.5, although the identification of a Y dwarf is somewhat surprising and may be indicative of either a flatter absolute magnitude/spectral type relation than previously reported or an upturn in the number of very late type brown dwarfs in the observed volume.